Oil Film, Foam on Surface of Tea (Tea Scum)
You might've noticed that sometimes when you steep tea, foam or an oil like film starts to collect on the surface. If it's the first time you see these bubbles/scum, then you could be worried that there's still some soap residue left in your teapot or cup. Or you believe that the tea isn't 'pure' or perhaps polluted.
Foam, Tea Scum
The truth is, when you see any foam collect on the surface, there's nothing to worry about. It's a result of a chemical reaction when hot water comes in contact with the leaves. The extraction of proteins and amino acids will result in foam.
Because these are healthy components in tea, some people argue that the more foam or scum you see, the more healthy the tea actually is. We won't go that far say that foam is a measure of quality.
If the layer on the surface looks more like an oily film, then it could be that you've a problem with the water that you're using. Most likely, you're dealing with hard water. When you cook hard water, there's a chemical reaction that results in substances that end up on the surface. Then when you steep tea leaves in it, the tannins of the tea will bind themselves on to this film substance, making them more visible.
There are no health risks using hard water, though it might not result in the best cup of tea. Try steep your tea with other water, and see if this solves the problem!